The present invention relates to the art of intrusion detection. More particulary, it relates to improvements in intruder detection systems of the so-called "dual technology" variety.
Heretofore, a variety of "technologies" have been used to detect the presence of an intruder in region under surveillance. Microwave, ultrasonic, photoelectric and passive infrared are some of the more common technologies in current use. Each has certain unique advantages and disadvantages which makes it more or less desirable for a particular environment or application. None is fool-proof, and all are subject to the ever-annoying false alarm.
In the never-ending struggle to provide the perfect intruder detection system, "perfect" from the standpoint that it never false alarms, proposals have been made to combine two (or more) technologies in a common intruder detection system. See, for example, the disclosures of U.S. Pat. Nos. 3,725,888; 3,801,978; 4,243,979; 4,275,390; 4,331,952 and 4,401,976. While such proposals go back at least twenty five years (see, e.g., U.S. Pat. No. 3,074,053), only recently has the cost of electronics reached a level that has made commercialization of a "dual-tech" system viable.
In conventional dual-tech systems, the outputs of the different intruder-detecting subsystems (e.g. microwave and passive infrared subsystems) are fed to an AND gate or its equivalent. Only in the event that the outputs of both subsystems indicate that both subsystems have detected intrusion substantially simultaneously, or within a predetermined time interval, will the AND gate provide an alarm-activating signal. The advantage of such a system, of course, is that false alarms will only occur on the relatively rare occasion that a spurious or false alarming-producing event is detected by both subsystems at about the same time. By combining relatively diverse technologies, e.g. microwave and photoelectric or passive infrared, the probability of false alarming can be minimized.
Heretofore, none of the proposed dual technology intruder detection systems has included any supervisory circuit for detecting a malfunction of either, or both, of the detection subsystems. It will be appreciated that, in the event only one subsystem fails, the entire system becomes inoperative since the AND gate requires two inputs, one from each subsystem, to activate an alarm. Worse yet, the inoperability of the system is not always readily detectable to the user. The problem of subsystem drop-out is particularly prevalent in dual-tech systems in which one of the subsystems is "active" in nature, e.g., conventional microwave, ultrasonic and beam-type photoelectric detection systems. In such systems, electromagnetic or acoustic energy is transmitted into a region under surveillance, and changes in the reflected or reflected energy are monitored. All too frequently, the transmitter (or receiver) element in such active systems fails and, without supervision, the entire dual technology system becomes inoperative.